How to concatenate char* with string

Official Hardware Retired GSM Shield
1

Hello guys,
i'm trying to make my GSM module send SMS to the user with the temperature value gotten from a sensor but i'm having issues with my code.
i tried to concatenate a char* with a string but its giving me this error: " unable to find string literal operator 'operator""String' with 'const char [52]', 'unsigned int' arguments
"
This is the code below:

#define DHTPIN 3     // Digital pin connected to the DHT sensor
#define DHTTYPE DHT11
#define PIN_TX    8 //pin TX for GSM module
#define PIN_RX    7 //pin RX for GSM module
#define BAUDRATE  9600
#define PHONE_NUMBER "+2349**23130**" //defining phone number of the user
//initial message sent to user after initialization is successfully completed.
#define MESSAGE  "System is online...\n Temperatue and Heat index:32.20°C +"temp"+ \n SELECT CROP TYPE \n 1: BEANS \n 2: RICE \n 3: YAM"
#define MESSAGE_LENGTH 5


char message[MESSAGE_LENGTH];
  int messageIndex = 0;
  int crops=0;
  int flag=0;
  
  char phone[16];
  char datetime[24];
  char temp[6];

temp is the string, I converted a temperature value from float to string using dtostrf function and the value was saved in "temp".

2

"char*" is a pointer.

You probably mean concatenate character strings, which you do in your program, not with #define.

Avoid using Strings with Arduino, as they cause memory problems and program crashes. Use the functions in <string.h> instead.

3

i tried to define the message it will send. and the value of temp will be sent with it.
Pls how can i use string.h to solve it, a sample should help.

4

You can do that entirely with Serial.print. Send messages like these to the GSM module:

Serial.println("System is online...");
Serial.print("Temperature is ");
Serial.print(temp);
Serial.print(" Heat index is ");
...
5

Or learn how to use sprintf.

6

i've solved the issue, thanks
your post here: Concatenate char* - #4 by jremington
it helped a lot.

7

It doesn't have anything to do with GSM Shield, anyway...

8

it does actually.
The main reason why I need to concatenate char* is because the GSM library I am using only allows char* to be sent as a text message to the user.
And the value of temperature is in float, I had to convert it to string using dtostrf function and save it to a char*.
Then I needed to concatenate it with another char* message, so the message can be sent together on the same line.

char temp[6];

dtostrf(t,6,2,temp);

  //using strcpy & strcat function to concatenate char* array (//buf is the initial message sent to user after initialization is successfully completed.)
  char buf[42];
  const char *first = "System is online...\nTemperature:";
  const char *second = temp;
  strcpy(buf,first);
  strcat(buf,second);
   
    
    Serial.println("start to send message ..."); //GSM module has initialized sucessfully, and will attempt to send a message.
    
    if (gprs.sendSMS(PHONE_NUMBER, buf)) { //define phone number and text
        gprs.sendSMS(PHONE_NUMBER, MESSAGE2);
        Serial.print("Send SMS Succeed!\r\n");
    } else {
        Serial.print("Send SMS failed!\r\n");
        delay(2000);
    }
9

Yes, you're right, but how to concatenate/convert char* ("C strings") and "String"s it's a general programming question, even if in this specific case you're applying it to GSM.

Anyway, I'm happy to see you solved it. HGC (Have a Good Coding)! :smiley:

10

Thanks :smile:

11

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